The present invention relates generally to rotary machines such as alternators for motor vehicles.
The single-phase or multi-phase generator constituted by a conventional motor vehicle alternator generally comprises a stator inside which there rotates a rotor provided with an excitation winding. The winding is powered by brushes in contact with two slip rings provided on a projecting portion of the shaft of the rotor.
Rotary machines are already known, in particular from EP-A-0 707 374, in which, in particular to increase their efficiency, the rotor excitation field is provided both by permanent magnets and by windings (this may be referred to as "combined" excitation), and in which the current delivered by the secondary is controlled by commutator means at the excitation windings, which commutator means make it possible selectively to reverse the excitation direction so as to reduce the flux from the permanent magnets or even make it substantially zero.
The need to reverse the excitation current direction makes it necessary to use an H-shaped semiconductor switching bridge whose cost is high and which thus pushes up the cost of the machine.
Electric motor structures are also known, in particular from FIG. 19 of Patent Application WO96/30992, in which the rotor is provided with:
at least two successive permanent excitation magnets which, in the rotor, generate two magnetic fluxes having components extending tangentially around the rotor structure in opposite directions in the direction of rotor movement; and
an even number of slots between the excitation magnets, together with windings that are wound in said slots and that are suitable for being powered in a full excitation direction or in a reverse direction so as to define alternating poles between said slots.
Those machines are not fully satisfactory insofar as slots between the magnets receive legs from two successive windings, each winding occupying only one half of the volume of a slot.
The fact that each slot between the magnets receives legs from two successive windings makes it necessary for the slots directly adjacent to the magnets to be filled only half-full. The ampere-turn distribution is thus asymmetrical in the slots of the wound poles; the slots directly adjacent to the magnets receive, in absolute terms, only one half of the absolute value of the ampere-turns received by the other slots.
That magnetic asymmetry modifies the usual distribution of the magnetic field lines (compared with wound machines having projecting poles, the machines have magnets only), thereby giving rise to premature saturation in the yoke or requiring the yoke to be over-dimensioned.
That is what is illustrated in FIG. 1a which shows a machine of the type proposed in Patent Application WO96/30992, together with the lines on which the magnetic fluxes are looped when the windings in the slots of the rotor are powered in their full excitation direction.